The invention relates to disc brakes, and is particularly but not exclusively concerned with such brakes as used with motor vehicles. It will be convenient to hereinafter describe the invention by reference to motor vehicle wheel brakes, but it is to be understood that the invention has wider application.
In a typical disc brake assembly for a motor vehicle, a caliper bridge is arranged to straddle a rotatable disc. The bridge incorporates inboard and outboard portions each of which incorporates an associated friction pad. The portions are able to move relative to one another to enable the friction pads to engage opposite faces of the disc under the influence of a hydraulic piston cylinder assembly acting on the inboard portion.
In operation, stresses are induced in the brake assembly which cause deflection of the caliper. In one form of deflection, the caliper may stretch in an axial direction, i.e. in a direction parallel to the axis of rotation of the disc. In another form, the caliper may yawn or splay under load causing uneven loading to be applied to the disc by the pad assemblies. Furthermore, the caliper may tilt relative to the axis of the rotation of the disc causing the pads to tend to taper after continued use. These deflections are undesirable as they increase the fluid required to actuate the brakes, cause variations in the specific torque output of the brake assembly under different loadings, and result in uneven wearing of the pads.
To compensate for the effects of this loading, different caliper designs have been proposed to minimise the deflection of the caliper. However, these designs have not been completely effective in stopping deflection of the caliper. Furthermore, these designs often add to the weight of the assembly, the complexity of its manufacture and the manufacturing costs. In addition these assemblies are often unable to dissipate heat as effectively due to the additional structure provided to make the brake assembly stiffer.
An aim of the present invention is to provide a disc brake assembly which is better able to accommodate the stresses induced in use of the brake assembly. It is a further aim of the invention to provide a disc brake assembly which has enhanced characteristics over prior art brake assemblies in relation to the cost of manufacture, its weight, serviceability and efficiency in dissipating heat.
In a first aspect, the present invention provides a disc brake assembly including, a disc having an axis of rotation and including a radially outer portion and a radially inner portion, a gap forming a radial separation between the outer and inner portions and extending in a continuous circle co-axial with the axis of rotation, two pad assemblies located on respective opposite sides of the disc and each being mounted for movement towards and away from the disc, and actuating means operable to cause each pad assembly to move towards and engage the respective adjacent side of the disc and including force transmitting means which extends through the gap and provides at least part of an interconnection between the pad assemblies such that one responds to movement of the other towards or away from the disc, wherein the actuating means includes a piston cylinder assembly in which the piston is movable relative to the cylinder along a movement axis, either the cylinder or the piston being connected to the other pad assembly for movement therewith, and the force transmitting means including a draw bar extending co-axial with the movement axis and connecting either the piston or the cylinder respectively to the pad assembly.
Preferably each pad assembly overlies part of each of the outer and inner disc portions and is engagable with the outer and inner portions on respective sides of the disc.
By locating the force transmitting means to extend through the gap in the disc, a caliper bridge is no longer required. Furthermore when each pad assembly is engagable with both the outer and inner disc portions, the brake assembly is better able to accommodate the loads induced in operation of the assembly as the bending moment on the force transmitting means generated by the contact surface which is at a radially outer side of the force transmitting means is offset by the bending moment generated by the contact surface at the radially inner side. Furthermore, with the draw bar extending along the axis of movement of the piston relative to the cylinder, bending stresses acting on the force transmitting means are minimised and the clamping load in the brake assembly is able to be transferred substantially along the force transmitting means. Consequently, the bending moment in the force transmitting means is reduced or even eliminated as compared to a conventional caliper bridge.
The individual pad assemblies include a section of friction material, with a surface of the friction material forming an operative surface of the respective pad assembly. Each pad assembly may incorporate separate sections of friction material, each of which has a discrete operative surface. However, for the purposes of this specification, the discrete operative surfaces of a respective pad assembly shall be collectively referred to as xe2x80x9can operative surfacexe2x80x9d for that pad assembly and may be considered as a single entity which has an outer perimeter defined by outer edges of the discrete operative surfaces.
The force transmitting means is operable to apply a force to a region of the operative surface of one of the pad assemblies at a location substantially coincident with the centre of that region. An advantage of invention is that as the draw bar extends through the pad assemblies, the draw bar can be connected to the other pad assembly at a position which provides a preferred load distribution on the support member of that pad assembly. In this way, deflection of the pad assembly in operation of the assembly may be minimised.
In a further aspect, the present invention provides a disc brake assembly including, a disc having an axis of rotation and including a radially outer portion and a radially inner portion, a gap forming a radial separation between the outer and inner portions and extending in a continuous circle coaxial with the axis of rotation, two pad assemblies located on respective opposite sides of the disc and each being mounted for movement towards and away from the disc, an actuating means operable to cause each pad assembly to move towards and engage a respective adjacent side of the disc and including force transmitting means which extends through the gap and provides at least part of an interconnection between the pad assemblies such that one responds to movement of the other towards or away from the disc, wherein the actuating means includes a piston cylinder assembly in which the piston is moveable relative to the cylinder, either the cylinder or the piston being connected to the other pad assembly for movement therewith and the force transmitting means includes a draw bar which extends along an axis and connects either the piston or the cylinder respectively to the one pad assembly, the draw bar being rotatable about its axis and being releasably coupled to the one pad assembly so as to be operative to move between an engaged configuration with the one pad assembly to a released configuration on rotation about the draw bar axis.
This arrangement has significant practical benefit as it allows for a secure connection between the draw bar and the one pad assembly which can be easily released or engaged by rotation of the draw bar about its axis, thereby contributing both to the safety of the assembly as well as its serviceability.
In a preferred form, the one pad assembly includes a passage adapted to receive a first end of the draw bar, the draw bar includes at or adjacent this first end at least one abutment surface disposed radially outwardly from the draw bar axis and which faces towards the piston cylinder assembly. The disc brake assembly is configured such that in the engaged configuration, the abutment surface engages with a complementary abutment surface of the one pad assembly so as to prevent removal of the draw bar from the passage and wherein when in the released configuration, the abutment surface is locatable within the passage so as to enable the draw bar to be released from the one pad assembly via the passage.
In a preferred form, the assembly further includes a retaining device operable to inhibit rotation of the draw bar when in engagement with the one pad assembly. Further, the assembly preferably includes biasing means to bias the draw bar towards the one pad assembly when in this engaged configuration.
The retaining device prevents unwanted draw bar rotation whereas the biasing means has a dual function of preventing rattle in the assembly as well as maximising the retraction of the one pad assembly on release of the brakes. The retaining device and the biasing means may be combined in a single component or separate components can be used for each device. Furthermore either of these devices can form an integral part of the one pad assembly or may be provided separately such as by a bolt on or a clip on feature.
In one form, the retaining device comprises a clip connected to or integrally formed with the one pad assembly, the clip includes a resiliently deformable body which includes at least one cavity adapted to receive a part of the draw bar. The cavity incorporates an inner surface which is engagable with the draw bar to inhibit rotation of the draw bar about its axis. In one form the biasing means is formed as part of the clip and includes an abutment member which abuts the draw bar so as to bias the draw bar into engagement with the one pad assembly. In one arrangement, the clip is caused to deform when the draw bar is captured within the cavity to apply the loading to bias the abutment member into engagement with the draw bar.
In a preferred form, the draw bar includes a plurality of abutment surfaces angularly spaced about the draw bar axis. In one form, the draw bar includes a T-shaped head having a pair of abutment surfaces spaced apart about the draw bar axis by approximately 180xc2x0.
Preferably, the one pad assembly includes a support member having opposite first and second major surfaces. A fiction lining is connected to or integrally formed with the support member along the first major surface to form an operative surface of the pad assembly and wherein the passage extends through the operative surface and at least into the support member. Preferably the passage extends through to the second major surface of the support member to thereby enable the first end of the draw bar to be accessed from the second major surface. Preferably in this arrangement, the first end further includes a drive coupling operative to transmit loading to rotate the draw bar about its axis.
In a preferred form of this aspect of the invention, the piston is movable relative to the cylinder along a movement axis and the draw bar extends co-axial with this movement axis.
Preferably, the brake assembly according to either aspect of the invention detailed above further incorporates an anchor bracket which is securable to a support structure and includes a body on which one of the pad assemblies is mounted to accommodate torque forces generated in use of the assembly. Preferably the bracket includes a pair of support elements which extend through the gap and are connected to the other pad assembly to at least assist in guiding movement of that pad assembly.
In a further aspect of the invention a brake sub assembly is provided for use in any of the forms of the disc brake assembly described above. The brake sub assembly includes the brake pad assemblies, actuating means and force transmitting means which is operable to pass through the brake pad assemblies to provide at least part of the interconnection between the pad assemblies.
In yet a further aspect the present invention provides a brake pad assembly for use in a disc brake assembly having an axis of rotation, the pad assembly including a support member and a friction lining connected to or integrally formed with the support member, a surface of the friction lining forming an operative surface of the pad assembly having relative to the axis of rotation, a radially outer and a radially inner edge interconnected by two side edges, the pad assembly incorporating at least one passage extending through said friction lining and at least into the support member, the or each passage being arranged to receive an end of a draw bar operable to impart an actuating force to said pad assembly, wherein the passage is of non circular cross section and the pad assembly includes at least one abutment surface disposed adjacent the or each passage which is operative to engage with a complimentary abutment surface of the draw bar to couple the pad assembly to the draw bar.
Preferably the brake pad assembly includes a retaining device operative to inhibit rotation of the draw bar relative to the pad assembly when in engagement with the abutment surface.
Preferably the support member includes a first major surface on which the friction lining is located and a second major surface opposite the first surface, and the passage extends through the support member to the second major surface. Preferably the passage includes a central axis and is configured to include a slot which extends radially to said central passage and which allows an end of the draw bar to be received in the passage. Preferably the abutment surface is angularly spaced from the radial slot so that the draw bar can engage with the abutment surface on rotation about the central axis of the passage.
In a preferred form, the passage is elongate having relative to its axis, a first radial dimension which is larger than an orthogonal second radial dimension. In this arrangement the radial slot extends in the direction of the first radial dimension and a pair of abutment surfaces are preferably spaced apart in the direction of the second radial dimension and disposed on respective opposite sides of the slot.
In a preferred form, the retaining device is in the form of a clip including a resiliently deformable body which includes at least one cavity adapted to receive a part of the draw bar, the cavity includes an inner surface which is engagable with the draw bar to inhibit rotation of the draw about the passage axis. Preferably the clip further includes an abutment member which is adapted to be biased into engagement with an end of the draw bar so as to bias the draw bar into engagement with the one pad assembly.
Preferably a region of the operative surface corresponds to the or each passage, each region being approximately a size proportional to an expected actuation load induced on the operative surface by the force transmitting element received within the respective passage and wherein, the or each passage is located substantially at the centre of that region. Further, the passage may extend to an edge of the pad to thereby facilitate lateral release of the pad from the force transmitting element.
The friction lining may also be separated into two parts by a space which extends circumferentially to the rotational axis of the disc with each part of the friction lining being engagable with a respective one of two radially separated portions of the disc.
Preferably each side edge of the support member also includes an abutment surface, these abutment surfaces being engagable with respective support elements of the disc brake assembly to transfer torque loading from the disc brake assembly to the support elements. In one form the abutment surfaces are located in recesses formed on the side edges of the support member. In another form, the abutment surfaces are formed on protrusions which extend from the side edges.
Preferably the friction lining also includes at least one radially extending channel extending across the operative surface to promote heat dissipation at the operative surface. Preferably the or each passage communicates with the channel or at least one of the channels formed in the friction lining.
In yet a further aspect the invention provides a disc for a disc brake assembly, the disc being rotatable about an axis of rotation and including a radially outer portion and a radially inner portion, each of the outer and inner portions having a braking surface on each of two opposite sides thereof and a gap forming a radial separation between said outer and inner portions and extending in a continuous circle co-axial with said axis of rotation, and connecting means spaced from the braking surfaces and interconnecting the inner and outer portions of the disc, the connecting means being in the form of a bracket located on one side of the disc, the bracket being spaced from the braking surface on that side of the disc to form a cavity in which a brake pad assembly is able to be located, the bracket further incorporating at least one passage providing access to the cavity.
Preferably the braking surfaces are provided on both the inner and outer portion and when used with a brake pad assembly as described above, the respective braking surfaces provide suitable engagement surfaces for the operative surfaces of the respective brake pad assemblies. Furthermore, the force transmitting means and the anchoring bracket are able to pass through the aperture in the disc, without inhibiting rotation of the disc.
Preferably there are a plurality of passages formed in the bracket. This arrangement also assists in ventilation of the assembly.
In a further aspect, the invention relates to a method of forming a disc of the above type. The disc may be formed in one piece or may be formed in separate pieces with the aperture and the contact surfaces machined by any or all of grinding, turning and milling of the disc